(1) Field of the Invention
The present invention relates to an xDSL (Digital Subscriber Line) transceiver using a discrete multitone modulation scheme utilizing the existing subscriber line as a high-speed data communication line, or in particular to an xDSL transceiver for realizing the suppression of an echo signal from a transmission unit to a receiving unit.
(2) Description of the Related Art
(i) Description of ADSL Technique
xDSL is known as a technique providing a digital subscriber transmission system utilizing an existing subscriber line as a high-speed communication line. xDSL is a transmission scheme utilizing a telephone line and one of the modulation-demodulation methods. It is roughly classified into two types according to whether the up transmission speed from a remote terminal (hereinafter referred to as a RT side) to a central office (hereinafter referred to as a CO side) is symmetric or asymmetric with the down transmission speed from the CO side to the RT side.
As one of the asymmetric xDSLs, ADSL (Asymmetric DSL) is known. ADSL may be a G.dmt type with the down transmission speed of about 6M bits/sec and a G.lite type with the down transmission speed of about 1.5 M bits/sec, both of which employ the DMT (Discrete Multitone) modulation scheme.
(ii) Description of DMT Modulation Scheme
FIG. 1 is a block diagram showing a configuration of a conventional ADSL transceiver. In the drawing, only the transmission unit on the CO side and the receiving unit on the RT side are shown, and the receiving unit on the CO side and the transmission unit on the RT side are not shown. The receiving unit on the CO side has substantially the same configuration as the receiving unit on the RT side shown in the figure, and the transmission unit on the RT side has substantially the same configuration as the transmission unit on the CO side shown in the figure.
The DMT modulation scheme will be explained with reference to the ADSL transceiver shown in FIG. 1 and taking G.lite as an example. This explanation refers to the modulation-demodulation in down direction from the CO to the RT. Nevertheless, the modulation-demodulation in up direction from the RT to the CO is carried out in a similar manner.
In FIG. 1, the transmission unit on the CO side includes a serial-to-parallel buffer (S-P buffer) 10 for converting serial transmission data to parallel transmission data, an encoder 20, a 256-point inverse fast Fourier transformer (hereinafter referred to as IFFT) 30, a cyclic prefix adder 40, a parallel-to-serial buffer (P-S buffer) 41, a D/A converter 50 and a transmission bit map 60.
The receiving unit on the RT side includes an A/D converter 80 for converting an analog signal from the subscriber line 70 to a digital signal, a time domain equalizer (TEQ) 90, a cyclic prefix remover 100, a serial-to-parallel buffer (S-P buffer) 101, a 256-point fast Fourier transformer 110, a frequency domain equalizer (FEQ) 120, a decoder 130 and a parallel-to-serial buffer (P-S buffer) 140.
Now, the operation will be explained. First, on the CO side, the transmission data is input to the ADSL transceiver and stored in one symbol time (4 kHz) in the serial-to-parallel buffer 10. The stored data is segmented into sections each having a number of transmission bits per carrier predetermined by the transmission bit map 60 and output to the encoder 20. In the encoder 20, each input bit string is converted into signal points for quadrature amplitude modulation and output to an IFFT 30. The IFFT 30 performs the quadrature amplitude modulation for each signal point by inverse fast Fourier transform and outputs the resulting signal to the parallel-to-serial buffer 41. In the process, a cyclic prefix adder 40 adds 240 to 255 samples of the output of the IFFT 30 to the head of the DMT symbol as a cyclic prefix. The output of the P-S buffer 41 is sent to the D/A converter 50 where it is converted into an analog signal at a sampling frequency of 1.104 MHz and, through a metallic line 70, is transmitted to the subscriber side.
In the receiving unit in the ADSL transceiver on the RT side, the analog signal is converted into a digital signal at 1.104 MHz by the A/D converter 80, and output to the time domain equalizer (TEQ) 90. In the TEQ 90, the signal is processed in such a manner that the inter-symbol interference (ISI) may be contained within the 16-sample cyclic prefix, and then stored in an amount corresponding to one DMT symbol in the S-P buffer 101. At the same time, the cyclic prefix is removed by the cyclic prefix remover 100. The outputs of the S-P buffer 101 are input to the FFT 110. In the FFT 110, a fast Fourier transform is performed to generate (demodulate) signal points. After that, the demodulated signal points are applied to the FEQ 120 where the effect on the amplitude and phase caused by the passage through the metallic line 70 is compensated for each carrier of a different frequency, and is decoded by the decoder 130 according to the receiving bit map 150 holding the same values as those in the transmission bit map 60. The decoded data is stored in the P-S buffer 140 and constitutes the receiving data as a bit string.
(iii) Explanation of the Echo Signal
FIG. 2 is a block diagram for explaining the echo signal in the conventional ADSL transceiver. In the drawing, unit #1 is one ADSL transceiver and unit #2 is the other ADSL transceiver. The transceivers are connected to each other by a subscriber line 240. Unit #1 includes a transmission unit 210 having the component elements designated by reference numerals 10 to 60 in FIG. 1, a receiving unit 230 having the component elements designated by reference numerals 80 to 150 in FIG. 1, and a hybrid circuit 220 for sending the transmission signal to the subscriber line 240 and delivering the receiving signal from the subscriber line 240 to the receiving unit. Unit #2 has the same configuration as unit #1.
The signal output from the transmission unit 210 of unit #1 is output to the subscriber line 240 through the hybrid circuit 220. The hybrid circuit 220 is so designed that the transmission signal thereof does not echo into the receiving unit 230. In an ideal hybrid circuit having a perfect impedance matching with the subscriber line, an echo of the transmission signal to the receiving unit 230 does not occur. Actually, however, the line characteristic varies from one subscriber line to another depending on the length, diameter, the condition of the bridge tap and the temperature, etc. of the subscriber line 240. Therefore, an impedance mismatch is caused in the hybrid circuit 220 so that the transmission signal from the transmission unit 210 echoes into the receiving unit 230 in the same system. This echo component constitutes a noise for the receiving unit 230 and is a cause of deterioration of the data transmission characteristics.
FIG. 3 is a diagram showing a spectrum of the transmission signal and the receiving signal in the conventional ADSL transceiver on the RT side. The spectrum of the transmission signal and the receiving signal of the ADSL transceiver on the CO side is similar to that of FIG. 3 and is not shown.
Conventionally, an effort has been made to remove the echo making up an echo signal from the transmission unit to the receiving unit using a sub-band filter. This conventional method will be explained with reference to FIG. 3.
In FIG. 3, a thin solid line indicates the receiving signal (down signal) from the CO side, and a two-dot chain indicates the transmission signal (up signal) from the RT side to the CO side. The receiving signal has a main component (in-band component) on the comparatively high-frequency band side and an out-of-band component on the comparatively low-frequency band side. The transmission signal, on the other hand, has a main component on the comparatively low-frequency band side, and an out-of-band component on the comparatively high-frequency band side. The receiving signal (down signal) is attenuated while passing through the subscriber line, and therefore in the drawing, is lower in gain than the transmission signal. As shown in FIG. 3, conventionally, in order to secure a sufficient S/N of the receiving signal against the out-of-band component of the transmission signal, a transmission low-pass filter (transmission LPF) for suppressing the out-of-band component of the receiving signal and passing only the main component of the transmission signal is arranged on the output side of the transmission unit 210, while a receiving high-pass filter (HPF) for removing the main component of the transmission signal and passing only the main component of the receiving signal is arranged on the input side of the receiving unit 230, thereby suppressing the echo signal from the transmission unit to the receiving unit.
For the S/N ratio of the receiving signal to be sufficiently large against the out-of-band component of the transmission signal, however, a transmission LPF and a receiving HPF having a characteristic with a sharp rise and a sharp fall are required, resulting in a large number of stages in the respective filters.
As explained above, in the conventional method using the transmission LPF and the receiving HPF, the order of the filter for suppressing the echo component must be increased and, therefore, the hardware including a filter, if any, is also increased in size. In the case where the filter is digitally configured with a digital signal processor (DSP) or the like, on the other hand, the processing amount is so increased as to require a DSP high in processing capacity. In either case, therefore, the system cost is increased. It is for this reason that inexpensive means for suppressing the echo from the transmission unit to the receiving unit is desired.